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Also on the no-fly list are “strong-jawed” dog breeds such as the American Pit Bull Terrier and Mastiffs.

Charles Hobart, a spokesman for United Airlines, tells PEOPLE the list of dog breeds the airline would not carry in cargo was previously six but has increased noticeably in an update made Tuesday. The new restrictions will be implemented on June 18.

“We are doing this to further minimize risk and ensure the comfort of pets we fly,” says Hobart. “Prior to today, we flew all sorts of animals. Geese, foxes, leopards, you name it, we pretty much flew it. That will change moving forward. We’ll only fly dogs and cats as pets that belong to our customers.”

Hobart says the airline’s priority is “the safety and comfort of those animals” and the dog breeds banned from traveling were at the recommendation of American Humane.

The new changes also mean United Airlines won’t fly pets between May 1 and Sept. 30 to four airports — Phoenix Sky Harbor International Airport, McCarran International Airport, Palm Springs International Airport and Tucson International Airport — due to high temperatures.

“We understand that [the new policies] can present challenges to folks who have traditionally flown their pets where they need to be, but our overwhelming concern is ensuring the comfort of those animals and this is how we have to do it,” he explains.

Hobart says customers preparing to fly with the United Airlines can prepare their pets for travel by “acclimating them to a kennel or crate” before flying out.

A decision on whether to implement restrictions for animals traveling in the cabin is pending further investigation, Hobart says.

PetSafe’s latest policy change comes after the airline began an investigation in March following the death of a French Bulldog in an overhead storage bin.

Raymond Boyd/Michael Ochs Archives/Getty

A passenger’s dog died on United Airlines flight #1284 after a flight attendant allegedly adamantly instructed the unnamed passenger to store her carrier — with the pet inside — in an overhead storage bin for the duration of the flight.

Maggie Gremminger, a passenger who witnessed the incident, told PEOPLE the black French bulldog barked from the bin at least 30 minutes into the flight, but was found dead when the owner went to retrieve her dog once the plane had landed.

“The flight attendant told the passenger that her bag was blocking part of the aisle. I could not see it, as I was already in my seat, but it sounded like it was somehow not completely fitting beneath the seat in front of her,” Gremminger said of the encounter.

“After the flight attendant asked her to move it above, the woman adamantly refused, communicating her dog was in the bag. There was some back and forth before finally, the flight attendant convinced her to move the carrier to the bin above.”

At the time, airline spokeswoman Maggie Schmerin told PEOPLE in a statement, “This was a tragic accident that should never have occurred, as pets should never be placed in the overhead bin. We assume full responsibility for this tragedy and express our deepest condolences to the family and are committed to supporting them. We are thoroughly investigating what occurred to prevent this from ever happening again.”

According to United’s pet policy online, non-service dogs are permitted in the cabin with a service fee of $125 as long as the dog “is in an approved hard-sided or soft-sided kennel. The kennel must fit completely under the seat in front of the customer and remain there at all times.”